Planning permission for a second pier was part of the original planning consent given in 1998 for the new terminal, east and northern aprons. Planning Permission has to be activated within 5 years though not necessarily completed in that time scale.
There was a separate application and permission for the new immigration and deep search link building.
It is debatable whether planning permission is required for the conversion of part of the short-term car park for aircraft stands. Last year the airside coach park by taxiway echo became stands 60 and 61. I remember reading some old minutes of the London Luton Airport Consultative Committee where it was raised that the stands didn’t need planning permission but maybe by moving the airside fence it does.